1. Field of the Invention
The present invention relates to a method for ascertaining the phase currents of an electric machine having a power converter as well as an arithmetic unit for carrying out the method.
2. Description of the Related Art
Generators for converting mechanical energy into electrical energy are known. In motor vehicles, claw pole generators are generally used. These may be equipped with electrical excitation. Since claw pole generators generate polyphase current, a rectification is necessary for the DC voltage vehicle electrical systems customarily used in motor vehicles. For this purpose, rectifiers having semiconductor diodes and/or active switching elements may be used.
Generators, which are also used for the vehicle drive, are known from the field of hybrid vehicles. These are to support the internal combustion engine in particular when it does not (yet) supply its full torque (boost operation, turbo lag compensation.) Motorized generators including their associated inverters are also referred to as electric drives within the scope of this application.
The regulation of an electric drive may be carried out with the aid of a field-oriented regulation in which the phase currents (i.e., currents through the phases or stator winding phases) of the machine are ascertained and regulated to a setpoint value. Usually two phase currents are measured in three-phase machines; the third phase current may be ascertained arithmetically, since the sum of all phase currents is zero.
Accordingly, in drives having more than three phases, N−1 phase currents must be measured, N being the phase number of the drive.
For the field-oriented regulation, the measured phase currents are transformed into a rotating-field-fixed dq coordinate system. It is not the individual phase currents which are important for the regulation, but rather the calculated current vector in the dq coordinate system. However, all phase currents must be known or at least arithmetically ascertainable.
The measurement of the phase currents may be carried out via measuring resistors, so-called shunts. Generally, these shunts are not directly installed in the phases of the drive, but in a low-side path of the power converter. For this reason, a measurement is only possible at points in time at which the phase current to be measured is negative, since only then are the diodes or active switching elements situated in the low-side path of the power converter conductive. For this reason, as a function of the activation (in particular PWM or block operation) of the power converter, only few fixedly defined points in time exist at which a current measurement may be carried out. This is not satisfactory.
The provision of measuring resistors having the required measuring electronics is moreover complex, so that simplified devices and methods for ascertaining the phase currents are desirable. In particular there is a need for such simplified devices and methods which may be used during PWM operation as well as during block operation.